Many WLAN systems implement IEEE 802.11, which is a family of standards offered by The Institute of Electrical and Electronics Engineers (IEEE). The IEEE standards specify the radio interface between access points and terminals and also between terminals. Currently, 802.11a, 802.11b, and 802.11g standards are widely used. Each 802.11 standard specifies a specific frequency band (e.g., 2.4 GHz for 802.11b/g communications and 5 GHz for 802.11a communications) at which access points and terminals can send and receive transmissions. Each frequency band is divided into channels. For example, the 802.11b and 802.11g standards define fourteen channels within the 2.4 GHz frequency band, as shown in the following Table 1.
TABLE 1Center FrequencyChannel(in MHz)AmericasEMEAJapan12412XXX22417XXX32422XXX42427XXX52432XXX62437XXX72442XXX82447XXX92452XXX102457XXX112462XXX122467—XX132472—XX142484——X
The use of certain channels by access points and terminals can be restricted in a geographic area by an applicable regulatory domain. For example, as shown in Table 1, the use of channels 12, 13, 14 is permitted (as denoted by the character “X”) in Japan by the Telecom Engineering Center (TELEC) regulatory domain, but is prohibited (as denoted by the character “—”) in countries in North, South, and Central America (the “Americas”) by the North American (FCC) regulatory domain. The use of channels 12 and 13 is permitted in countries in Europe, the Middle East, Africa, and various parts of Asia (“EMEA”) by the European Telecommunications Standards Institute (ETSI) regulatory domain, but the ETIS regulatory domain prohibits the use of channel 14 in EMEA.
Similarly, the IEEE 802.11a standard also defines channels within the 5 GHz frequency band, the use of which is also regulated differently by the TELEC regulatory domain, the North American regulatory domain, and the ETSI regulatory domain, as shown in the following Table 2.
TABLE 2Center frequencyChannel(in MHz)AmericasEMEAJapan345170——X365180XX—385190——X405200XX—425210——X445220XX—465230——X485240XX—525260XX—565280XX—605300XX—645320XX—1005500—X—1045520—X—1085540—X—1125560—X—1165580—X—1205600—X—1245620—X—1285640—X—1325660—X—1365680—X—1405700—X—1495745X——1535765X——1575785X——1615805X——
Regulatory domains also regulate the maximum transmission power allowed per antenna gain. Antenna gain relates the intensity of an antenna in a given direction to the intensity that would be produced by a hypothetical ideal antenna that radiates equally in all directions, isotropically, and has no losses. Table 3 depicts maximum power levels per antenna gain in accordance with the 802.11g standard and as regulated by the TELEC regulatory domain, the North American regulatory domain, and the ETSI regulatory domain.
TABLE 3Ameri-Ameri-cas -cas -EMEA -EMEA -Japan -Japan -MaxMaxMaxMaxMaxMaxPowerPowerPowerPowerPowerPowerLevelLevelLevelLevelLevelLevelAntenna(mW) -(mW) -(mW) -(mW) -(mW) -(mW) -Gain (dBi)CCKOFDMCCKOFDMCCKOFDM2.2100305030556100303010556.51003020105510100301055513.510030555515502051552120101—55
A terminal operating in a regulatory domain over permitted channels and maximum transmission powers can use two types of scanning to search for an access point of a WLAN with which to associate. In passive scanning, the terminal generally listens for beacon frames broadcast by access points, one channel at a time. The beacon frames include the service set identifier (SSID) of the WLAN being hosted by the access point. Since beacon frames are often broadcast at regular intervals of approximately 100 ms, the terminal typically has to “dwell” for about 105 milliseconds (ms) on a first channel before trying a second channel. The lack of a beacon frame over the first channel can mean either that there are no access points broadcasting on the first channel or that the first channel is outside of the legal frequency band of the regulatory domain. By listening to all channels that are used in the WLAN via passive scanning, the terminal can collect all the information on channels that are currently supported by access points and within the legal frequency band of the regulatory domain.
By contrast, in active scanning, the terminal transmits an active probe request on a channel. The terminal may receive one or more probe responses from access points. The probe responses may be received within 15 ms of the transmission of the active probe request, thereby making active scanning relatively faster than passive scanning. If no probe response is received in that time, there are either no access points on that channel, or the channel is outside of the legal frequency band of the regulatory domain. The terminal may then try a different channel.